According to an ultrasonic diagnostic apparatus, a voltage is applied to a transducer which is mainly constructed by a piezoelectric material, an ultrasonic wave generated from the transducer is transmitted to an object, and various information is taken out from a reflection wave of the ultrasonic wave, thereby obtaining information of the inside of the object.
Ultrasonic image quality is dependent on how excellently ultrasonic wave transmission/reception beams can be formed over all depths. The wave reception beam has higher precision by digital phasing and can implement dynamic focus. On the other hand, the wave transmission beam must sacrifice a frame rate thereof in order to implement dynamic focus, and thus it is required to form an excellent beam having high spatial resolution in one beam formation. That is, it is required to lengthen a focal depth and suppress a side lobe. Therefore, it is indispensable to weight a time direction of a waveform of a transmission wave and an aperture direction used for beam formation.
Formation of an amplitude-weighted waveform which is effective to suppress the side lobe, enhance a contrast spatial resolution, etc. by a digital phasing technique has been recently facilitated even in wave-transmission phasing.
A linear wave-transmission amplifying circuit for precisely amplifying any transmission signal of minute amplitude output from a DA converter to a high voltage is generally necessary to drive a transducer by using the weighted waveform described above and generate an ultrasonic wave. A transducer disposed in a probe has an action of converting an electrical signal to an ultrasonic signal. In order to form an ultrasonic beam and obtain sufficient information for diagnosis, it is necessary to apply a signal of about one hundred and several tens volts and thus it is necessary to amplify the output from the DA converter until this level.
In general, in the ultrasonic diagnostic apparatus, the linear wave-transmission amplifying circuit is frequently implemented by a P-channel (hereinafter referred to as Pch) and an N channel (hereinafter referred to as Nch) of a field effect transistor (Field Effect Transistor: FET) or a transistor which enables flow of current whose amount is necessary to drive a probe having a relatively low impedance or by joining NPN and PNP types using these transistors. In order to simplify the description, the description will be described by specializing FET, however, the same effect can be obtained by electrical elements having a current amplifying action such as a transistor, etc.
With respect to FET, various circuit parameters such as capacity, ON-resistance under operation, etc. are different between the Pch and the Nch. Therefore, in order to compensate for this as much as possible and implement a linear amplifying operation irrespective of the magnitude of an input signal, it is necessary to make bias current of about several mA flow steadily even when no signal is input.
Furthermore, as described above, a maximum applied voltage of about hundred and several tens volts is required to drive the probe as described above, and thus for example, about ±100V is required to a power supply at a device side. The bias current described above is required per channel, and it is consumed in the power supply of about ±100V. Therefore, heat generation of several W to several tens W occurs necessarily as the whole of the ultrasonic diagnostic apparatus.
As described above, in order to enhance the spatial resolution of diagnostic images, a wave-transmission circuit which can perform the linear amplification operation is required, and large power consumption is required as the whole device to implement this circuit system.
Plural attempts to reduce this large power consumption have been proposed. According to Patent Document 1, an ultrasonic diagnostic apparatus is provided with means for determining, as power save units, operation-unnecessary units and operation-limitable units out of plural units constituting the device in accordance with an operation condition of the device when a diagnostic image is formed, and the operation of each unit is limited at a proper timing to suppress power consumption.
Furthermore, according to Patent Document 2, a power supply voltage value to be supplied to a transmission circuit is switched to another fixed voltage source or another control voltage source in accordance with a transmission voltage supplied to an ultrasonic probe in the transmission circuit, whereby the power consumption in the transmission circuit and the heat generation caused by the power consumption are reduced.